In the same way as in the visible light optical range, the X-ray optical range must also convert a phase contrast to an amplitude contrast to make a phase contrast object visible. Therefore, a phase contrast image means a graphical representation of a phase contrast converted to an amplitude contrast. However, images of objects are considered throughout in which the contrast image is actually based on phase contrast and not amplitude contrast.
The phase contrast radiography underlying the invention is based on the fact that X-rays which pass through a phase contrast object, i.e. through adjacent ranges of different optical thickness, have a well-defined phase difference to one another. Therefore, these X-rays can interfere with one another (X-ray interference). As a result of this X-ray interference, an amplitude or intensity contrast image is observed at a sufficient distance. The interference is also related to a deflection of the radiation to the direction of incidence (diffraction). The above-mentioned phase contrast object can be seen as a transparent object with one lateral variation of the thickness, the refractive index or both. In contrast to the X-ray absorption radiography, an image of an object can be generated with the phase contrast radiography which has a lower absorption for X-rays and small absorption contrasts based on the thickness, the density or the element composition.
A phase contrast X-ray device of the kind mentioned at the beginning and an appropriate method is e.g. known from Wilkins et al., Nature, 384 (1996), pages 335–338 (cf. FIG. 2). The X-ray source of the known X-ray device is point-shaped and has a very small diameter from 5 μm to 15 μm. The evaluation unit is, for example, an X-ray film. The object to be investigated is arranged within the optical distance to the X-ray source between the point-shaped X-ray source and the evaluation unit. The optical distance results from a ray path of the X-radiation. Divergent X-rays radiated from the point-shaped X-ray source pass through the object. At a phase limit of the object, a passing through of the object causes a phase shift of the X-radiation. Both phase-shifted and non-phase shifted X-rays reach the evaluation unit, are converted to an amplitude contrast there and made visible as a so-called phase contrast image.
Based on the smaller diameter of the point-shaped X-ray source of the known phase contrast X-ray device, a (radiographic) output of the X-ray source is restricted to below 50 W. Because of the lower output, the phase contrast X-ray device is suitable for creating a phase contrast image of a thin, small object, for example an insect. The known phase contrast X-ray device is not suitable for larger and thicker objects, for example a human being, because of the lower output. Therefore, the phase contrast X-ray device is also not suitable for use in medical technology.
A monochromator as a gradient multilayer reflector is known from Schuster et al., Proc. SPIE, 3767 (1999), pages 183–198. The gradient multilayer reflector is an artificial, one-dimensional grid that allows the Bragg area of reflection of X-radiation. The reflector distinguishes itself by means of a periodic series of layers of a first layer type A and a further layer type B. The first layer type A has a first refractive index rA and a first layer thickness dA and a further layer thickness B, a further refractive index rB and a layer thickness dB differing from the first refractive index rA. In one lateral direction of propagation of the reflector, the layer thicknesses increase by a total of d=dA+dB. The gradient multilayer reflector then has an area of reflection that can be elliptical, parabolic, circular or planar.
The gradient multilayer reflector is used, for example, as a mirror in X-ray diffractometry. By using this gradient multilayer reflector, parallel and nonparallel X-radiation of a relatively great photon energy bandwidth can be reflected and can be monochromated with a relatively small intensity loss.